Tall oil pitch is obtained from the black liquor of alkaline digestion of coniferous wood, most notably the kraft process. The black liquor is typically concentrated and settled to yield soap skimmings that contain sodium salts of fatty acids, sodium salts of resin acids and unsaponifiables. The latter group of substances include fatty alcohols, free sterols, steryl esters, and fatty acid esters. In kraft pulp mills, the collected soap is routinely acidulated with a mineral acid such as sulphuric acid to yield an oil phase and a water phase. The oil phase contains free fatty acids, resin acids and unsaponifiables; it is commonly known as crude tall oil. Typically, the amount of unsaponifiables can range from 10 to 35% by weight of the crude tall oil, depending on the species and quality of coniferous wood used. The water phase containing sodium sulphate and any lignin entrained in the original soap is normally recycled back to the pulp mill chemical recovery system. In the subsequent recovery of desired fatty acids and resin acids, crude tall oil is typically evaporated under low pressure conditions to yield a light phase, known as depitched tall oil, containing mainly fatty acids and resins, and a heavy phase, known as tall oil pitch, containing a small amount of fatty and rosin acids and a substantial amount of the original unsaponifiables.
Phytosterols can be isolated from either tall oil soap (sometimes referred to as soap skimmings) or from tall oil pitch. It is understood that the manufacture of sterols from tall oil soap has been practiced commercially by Oy Kaukas AB, Lppeenranta, Finland since 1981. The technologies are those based on the refining of tall oil soap with a combination of low molecular weight ketones, alcohols and hydrocarbons; for example, as disclosed by Holmbom et al. in U.S. Pat. No. 3,965,085 granted on 22 Jun. 1976. The refined tall oil soap is then extracted and crystallized using a combination of polar and non-polar solvents, for example, as taught by Johansson et al. in U.S. Pat. No. 4,044,031 granted on 23 Aug. 1977 and Hamunen in U.S. Pat. No. 4,422,974 granted on 27 Dec. 1983. Those methods of manufacture of pure tall oil sterols requires the soap skimmings to be relatively free of entrained black liquor and the use of multiple solvents which entails several separate solvent recovery systems. The adjustment of precise solvent compositions to maintain optimal operation for each processing stage is complex. In U.S. Pat. No. 4,153,622 granted on 8 May 1979, Lamminkari et al. disclose the use of acetone and activated carbon to extract sterols from tall oil soap, in which the acetone extract is subsequently evaporated for dissolution in ethanol for the final recovery of sterols.
The recovery of sterols from tall oil pitch has been studied for many years. In U.S. Pat. No. 2,715,638, Albrecht et al. teach the use of an amount of dilute alkaline solution to neutralize the fatty and rosin acids in tall oil pitch but in an amount to saponify the sterol ester. The remaining organic phase is then separated and saponified with an alcoholic alkaline solution to convert steryl esters into free sterols for subsequent dilution in hot water to precipitate the sterols by cooling. The product purity was indicated to be in the range of 83%. In U.S. Pat. Nos. 3,691,211 and 3,840,570 Julian teaches the use of a mixture of alcohol, water and hydrocarbon to extract tall oil pitch, then saponify the hydrocarbon phase with an alkali metal base, and finally dissolve the saponified material in a polar solvent for the recovery of phytosterols. The procedure is cumbersome as it involves several solvent extraction steps with different polar and non-polar solvents. The solvent recovery systems for at least polar and non-polar solvents are complex.
In U.S. Pat. No. 5,097,012 granted on 17 Mar. 1992, Thies et al. disclose a method for the isolation of sterols from crude tall oil by water extraction at elevated temperatures and pressures.
In U.S. Pat. No. 3,943,117 granted on 9 Mar. 1976, Force discloses a process for saponifying tall oil pitch in which a water-soluble cationic amine is used in conjunction with an alkali. In U.S. Pat. No. 4,524,024 granted on 18 Jun. 1985, Hughes teaches the hydrolysis of tall oil pitch at elevated temperatures to increase the recovery of fatty acids from tall oil pitch. In U.S. Pat. No. 3,887,537 granted on 3 Jun. 1975, Harada et al. disclose the recovery of fatty acids and rosin acids from tall oil pitch by first saponifying tall oil pitch with an alkali metal base and a low molecular weight alcohol, and then introducing the reacted mixture into a thin film evaporator to remove low-boiling matter such as water, alcohol use and light unsaponifiables. The bottom fraction from the first evaporator is next fed to a second thin film evaporator in which the unsaponifiables including sterols are removed as the light ends and a molten soap is recovered as the bottom fraction. Fatty acids and rosin acids are recovered from the molten soap fraction by acidulation conventionally with a mineral acid. In U.S. Pat. No. 3,926,936 granted on 16 Dec. 1975, Lebtinen teaches the recovery of fatty acids and rosin acids from tall oil pitch by reacting tall oil pitch with an alkali at 200 to 300 degrees Celsius, in the amount of 5 to 25% of tall oil pitch, prior to vacuum distillation of the heated mixture to recover the fatty acids and rosin acids in the distillate fraction.
Reference is also made to Chemical abstracts, vol. 112, no. 20, 14 May 1990, Columbus, Ohio US; abstract no. 181758, MALIK, Lubomir et al: “Isolation of phytosterols from tall—oil rosin”, XP002104877 & CS 256 092 A (Czech). Malik et al. disclose a process for extracting phytosterols which includes the use of four distillation stages. Product flow is split into parallel distillation paths in a first distillation stage then, following further distillation in each parallel path, is partially recombined prior to a final distillation stage. To achieve high purity phytosterols, the output from the final distillation stage is subjected to two stages of crystallization utilizing relatively large amounts of solvent.